The present invention is directed to lithography, and in particular, to compensating for deformities in color separations by imaging correspondingly deformed images on printing plates, such that these plates will print the color separations in register.
Conventional lithography processes typically replicate images by transferring ink from a previously prepared plate onto a substrate, typically paper. In offset printing, this transfer is done indirectly by a soft blanket that is stamped by the plate. This soft blanket then impresses this image onto paper.
Color images are obtained by separating the image into four color process plates, these four colors corresponding to Cyan, Magenta, Yellow and Black, commonly known as “CMYK”, that are then combined on paper. If an image is to be accurately represented, all four color separations must have the same length, scale and position when impressed on the paper. However, this is difficult, as there may be misalignment and scale errors, these errors being classified as either fixed errors or paper errors.
Fixed errors are errors in misalignment and scale that do not change from image to image. These errors are typically caused by tolerances of the prepress imaging devices, errors produced during development of the imaged plates, errors in mounting the plates on the printing press cylinders, and errors of the printing press mechanisms. Errors of the printing press mechanisms typically include plate cylinders having lost their roundness from wear or the like, and loosening of gears, bearings, etc., typically from wear over time.
Paper errors typically result from paper wetting by fountain solution and ink, and by forces applied on the paper by the printing press, that tend to deform the paper. With the paper deformed, data is printed at undesired or unintended locations. Even when the deformation forces on the paper are released, the paper does not usually recover to its original configuration, and thus, there is a difference between the data (on the imaged printing plate) and the resultant printed image.
Paper deformation effects, that result in paper errors, are explained by FIGS. 1A-1C Specifically, FIG. 1A shows the desired or ideal situation where a rectangular image 20, formed of lines 20a and 20b, is to be imposed on the substrate 22, typically paper. In FIG. 1B, the paper 22 has been impressed with a first separation, here a black (K of the CMYK) impression 24. This impression 24 is trapezoidal in shape (formed of lines 24a, 24b) as a result of the paper deformation in the press. Subsequently, in FIG. 1C, a second separation, for example a Cyan or “C” separation is impressed onto this deformed paper 22, as represented by broken line 26, in a misregistration. Similarly in this manner, the subsequent Yellow “Y” and Magenta “M” separations will also be misregstered in accordance with the paper deformation.
In conventional press systems, some of the misregstration caused by the fixed errors was correctable mechanically by the operator after reviewing the initially printed sheets. In this case, the operator manually adjusted the relative positions of the printing plates or changed pressure on the printed substrate.
These manual, operator-made adjustments have drawbacks. Initially, these adjustments are time consuming and require considerable operator skill. Additionally, the adjustments of plate cylinders required expensive mechanical devices. Even in conventional presses, although the time required for adjustment is lessened, the adjustment machinery is more complex and more expensive.
A few presses have been designed such that printing plates are imaged on the press, or “on-press”, whereby the plates are not removed from the press for imaging. One exemplary press is the Model 74 KARAT offset digital press, manufactured by a Karat Digital Press of Herzlia, Israel in a joint venture with KBA (Koenig & Bauer Aktlengesellschaft). This design, besides reducing the make ready or preparation time, allows elimination of most of the registration fixed errors. However, paper errors remain and to date, a procedure does not exist for correcting misregistration caused by paper errors.